Glyphosate Resistance Spreads in the U.S.

Glyphosate Resistance Expands in U.S. Acres

First reported in 1998 in a pair of California almond orchards, glyphosate resistance is spreading in the United States, and the weeds are only getting smarter. The problem has intensified as multiple weed species are now resistant on a growing number of farms, according to an extensive new survey.

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Kent Fraser, vice president of the research firm Stratus Agri-Marketing Inc., which conducted a survey of thousands of U.S. growers across 31 states over three years, told Farm Chemicals International, “We were surprised to see how quickly the problem of glyphosate resistant weeds has spread, especially in the Midwest. It is also alarming that the number of farms with more than one of the confirmed resistant species has increased so fast.”

The crop input industry has responded with products of differing modes of action designed for resistance management. The products are to be used as part of an integrated approach with crop rotation representing a key component. “It will be interesting to see if growers choose to rotate chemistries from year-to-year, mix it up with multiple modes of action at different application times in one year, or use products with multiple modes of action built in,” Fraser said.

U.S. growers told Stratus that 61.2 million acres of cropland are infested with glyphosate resistant weeds — almost double the number recorded in 2010.

According to the International Survey of Herbicide Resistant Weeds, there are currently 396 unique cases of herbicide resistance globally, with 210 species. Herbicide-resistant weeds have been reported in 63 crops in 61 countries.

The survey’s results were dramatic:

  • Nearly half (49%) of all U.S. growers said they have glyphosate-resistant weeds on their farm in 2012, up from 34% in 2011.
  • Resistance is still worst in the South. For example, 92% of growers in Georgia said they have glyphosate-resistant weeds.
  • The mid-South and Midwest states are catching up. From 2011 to 2012 the acres with resistance almost doubled in Nebraska, Iowa and Indiana.
  • It’s spreading at a faster pace each year: total resistant acres increased by 25% in 2011 and 51% in 2012.
  • The problem is getting more complicated. More and more farms have at least two resistant species on their farm. In 2010 that was just 12% of farms, but two years later 27% had more than one.
  • Marestail (horseweed) was the weed most often reported with glyphosate resistance, followed by Palmer amaranth (pigweed). Another half-dozen species were tracked in the study.